This invention relates to decurlers suitable for taking the curl out of sheet material in web or roll form such as paper and paperboard. Curl in paper results in the tendency of the sheet material to curve or warp due to stresses or other factors within the material. For instance roll curl is caused from the material being wound into rolls for storage and material shipment. This curl runs along the web or grain in the direction known as the machine direction.
Another factor causing curl in sheet material is coating on the flat surface of one side. If the paper is coated on one surface or coated more or differently on one surface than the other in a manner such that moisture penetrates into one surface more easily than the other, there results a curl in the direction crosswise of the web due to the uneven swelling and expansion causing tension in the material as the moisture is absorbed.
Prior decurling apparatus of the general type to which the subject invention pertains are shown in U.S. Pat. Nos. 2,399,070, Sheet Decurling Apparatus, issued on Jan. 11, 1944; 2,531,619, Machine for Decurling Labels, issued on Nov. 26, 1950; 3,661,703, Decurling Apparatus, issued on May 9, 1972 and 3,799,038, Curl Corrector Apparatus for Operating on a Continuous Web, issued Mar. 26, 1974.
Such prior decurling apparatus have been used in an attempt to remove the curl in an advancing web by positioning the members which deflect the web transverse of the direction of travel. Such apparatus has primarily only removed the curl running lengthwise of the web. It is the purpose of the present invention to remove both lengthwise and crosswise curl in an advancing continuous web.